There have been numerous devices invented for controlling the level of fluid in containers and among these are several electronic devices exemplified in U.S. Pat. Nos. 3,757,317 (Kahn et al), 3,978,352 (Rose), and 3,787,733 (Peters). In the prior art, electronic devices for controlling the level of fluid in a container or tank between two predetermined levels utilized electronically active devices such as transistors involving a fair amount of expense in parts and production. In the prior art, exemplified by the above-mentioned patents, electronic circuits usually serve the function of a latch control device which caused the mechanism controlling the fluid level in the tank to operate when the fluid exceeded a predetermined level.
One method was found in the prior art which eliminated the apparent necessity of three transistors to form a latching mechanism. This method is exemplified in U.S. Pat. No. 3,894,240 (Rose), which discloses a latching circuit making use of the voltage differential between the ignition voltage and the conductance voltage of a neon bulb. In the device, exemplified by the Rose '240 patent, the neon bulb is connected across a power source through two probes and a voltage dividing circuit. The voltage divider must be precisely adjusted such that the voltage difference between the two probes is equal to the voltage difference between the ignition voltage and conductance voltage of the neon bulb. These two probes define an upper level and a lower level in a fluid container. The lamp is ignited whenever fluid reaches the level of the upper level probe and remains ignited until fluid drops below the level of the lower level probe. Because of the fact that both the ignition voltage and the conductance voltage of such a neon bulb change with the aging of the bulb and with changes in temperature, and also vary among different neon bulbs, the adjustment of the voltage divider circuit is critical in order to maintain the voltage differential. Thus, in such a device, it would be expected that frequent adjustments of the voltage divider circuit would be necessary to account for the age of the bulb, temperature changes, changes in the resistance of the fluid, and any necessary change in neon bulbs. Devices for controlling fluid level using a neon bulb and photosensor, which depend upon the voltage differential between the ignition and conductance of a neon bulb, have an inherent source of unreliability due to the necessity of adjusting the voltage divider circuit of the device.
Thus, the prior art never completely solved the problem of finding a simple, reliable control circuit for controlling fluid level between two points in a tank.